The present invention relates to a new and improved construction of a gas-blast switch.
The gas-blast switch of the invention is of the type comprising a gas tight housing incorporating a hollow insulator. Within the housing there is arranged a set of fixed contacts and a set of movable contacts surrounded by a blast chamber as well as a blast nozzle operatively associated with the set of movable contacts and connected with a pump device. This pump device embodies a pump cylinder surrounding a pump chamber and movable in conjunction with the movable set of contacts. The pump cylinder is displaceably mounted upon a stationary piston. A nonreturn or check valve merging with the pump chamber and opening towards the pump chamber sucks-up pressurized extinguishing gas into the pump chamber during the course of the cut-on stroke of the gas-blast switch.
Such switches are designed as so-called "lifetank" switches, wherein the housing carrying the electrical connections of the switch and predominantly composed of the hollow insulator is attached to a support insulator. The entire requirement of extinguishing or quenching gas is withdrawn from the contents of the switch housing itself, including the contents of a possibly provided support tube and a drive housing. In contrast hereto, in the case of the so-called "deadtank" switches there is provided a grounded metal housing. The requirement of extinguishing or quenching gas, in this case, is furnished by the contents of the relatively voluminous metal housing. Here the electrical connections are conducted out of the metal housing through electrical passages.
The switches of the initially mentioned type, thus, in comparison to the so-called "deadtank" switches, contain an appreciably smaller quantity of extinguishing or quenching gas, for instance SF.sub.6.
The blowing of the arc during the course of the cut-off stroke causes in the blast chamber or compartment--in other words in a spatially limited region--an appreciable pressure and temperature increase. While the pressure increase tends to propagate to all parts of the switch practically without any delay, the temperature increase initially is limited to the region of the blast chamber and temperature compensation by means of the gas at the more remote locations, such as for instance at the drive housing, only occurs gradually. If there is considered that the quantity of gas in the switch is constant, it will be apparent that at that location where there prevail increased gas pressure and also an appreciable temperature increase governed by the arc, there is present lower gas density, than at that location where initially there has been effective the pressure increase, and possibly a low temperature increase due to the compression.
Now if, as is the rule with a heretofore known switch of the previously mentioned type, for instance as taught in French Pat. No. 2,291,601, following a cut-off or switch-off operation, caused by a short-circuit condition, there is again automatically triggered a cut-on or switching-in operation after several tenths of a second, then in the pump chamber or compartment gas will be sucked-up from that region where the density of the gas has decreased owing to the previously mentioned cut-off operation. Now if following the automatice cut-on or switching-in operation the short-circuit has not been eliminated and there directly follows a second cut-off operation, approximately 0.3 seconds after the first cut-off or switching-off operation, then there is only available in the pump chamber gas having a low density for blowing the arc which has formed during the course of this cut-off operation. Gas of lower density however means lower dielectric strength and reduced cut-off efficiency, and equally--since during the second cut-off operation there has not yet occured any appreciable cooling of the gas--reduced cooling capability and at the same time approaching the ionization temperature of the extinguishing gas. Thus, with a " lifetank" switch of the previously mentioned type there must be reckoned with a quite appreciable reduction of the cut-off efficiency when there are undertaken brief successive cut-off or switching-off operations.